Fig. 4
From: Fin modules: an evolutionary perspective on appendage disparity in basal vertebrates
Hypothesized scenario for the evolution of median and paired fins. Both median and paired fins developed first as elongated ribbon-like structures (a) that are gradually modified into short-based fins (b). Serial duplications of fin modules lead to the emergence of novel fins such as the pelvic fins or a second dorsal fin (c). Divergence or co-option of some fin modules also leads to the evolution of novel fins, such as the adipose fin of euteleosts or the spinous dorsal fin of acanthomorphs (d)